Many people can grow extremely long head hair and facial hair. Are there evolutionary theories as to why this is the case? It seems like having long hair could be a disadvantage, and extremely long hair seems to be rare in other mammals.

Just want to point out that not everything can be explained in terms of advantage or disadvantage in evolution. Most of the characters are not directly selected or selected at all.
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biojlJan 7 '13 at 15:25

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@nbubis: why has there to be an advantage? Maybe it's just that it is not disavantageous...
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nicoJan 7 '13 at 19:10

2 Answers
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How humans evolved to have head / beard hair that continues to grow longer than other animals is a topic that many anthropologists & biologists are still not sure about & there is no general consensus as to "why" yet.

The three main views that I am aware of however are :

1) Evolution of the "Aquatic Ape." (Ingram, 2000: Morgan 1997; 1982)

Infants, in order to hold onto their mothers in the water, would latch onto her hair. Limiting separation from the mother & increasing chances of survivability

Longer hair meant that infants / small children would need to swim less in order to get to their mother

Believed to be supported even further when you consider that aquatic mammals are almost always hairless, indicating that at one point, humans were highly "aquatic" mammals.

2) No real benefit, but used as a tool for "mate selection." (Darwin, 1871; Cooper 1971)

The view held by many of the Darwin school of thought (I do not mean that as a derogatory), is that at first, "hairiness" was sexually attractive, but eventually "hairlessness" became more sexually attractive in most places (i.e. the face to see facial expressions & socialize better; Wong & Simmons 2001)

A sign of "virility" & "health" as can be seen in the mate-selection behavior of lions. Which is true even today as human diagnostic material for health (Klevay, 1972).

3) Practical evolutionary benefits for the human species specifically

A lot of body heat escapes from the head, probably the most important part of your body. Hair is a good insulator that can keep in heat. This increases survivability in colder climates. (Wong & Simmons 2001; Bubenick 2003). (A disputed but considered credible reason, especially when you compare hair length and types across different regions throughout history)

Protection against damaging UV rays (while still permitting adequate Vit.D3 to come through) & some protection from free-radicals or other harmful particles. Because we became bi-pedal, the head was the main area exposed to the sun (as well as some of our back). Extending hair's usefulness to even hot environments, while other body hair became less important with the development of sweat glands (Wheeler 1985).

Heightened "Situational Awareness" through "Touch sense." A concept that may seem silly at first but has some evidence to support the theory. Though the hair is not "alive," it is connected to the follicles & your nerves. In a nutshell, it may help to increase "sensory awareness" & "data gathering" of your environment, which would favor longer hair. This would be an asset in survivability (Kardong 2002; keratin.com 2010; Sabah 1974; washington.edu)

It's important to notice that there isn't a set point for the actual hair length. The hair doesn't know that it's been cut for example. Rather, there's a set point for time. A single strand of hair, in humans, on the scalp, will be in the growth phase for several years. The growth cessation takes a few weeks, and then the rest phase will last for another couple of months, until finally the hair strand exits.

I don't think there's a natural advantage for hair of as long a length as many humans can grow it, it may just be little or not at all disadvantageous. The selective advantage, if there is one, probably eminates from sexual selection, similar to why lions grow manes. A display of health and surplus of resources. Hair has a pretty central role in beauty ideals. In some cultures, women even cover up their hair.

There are examples of traits that are disadvantageous to survival in the organism's environment, but shows advantages when it comes to reproduction, to a degree that the genes do not become extinct. I don't remember the name, so take it for what you will, but there's a fish species where the males are either small and stealthy, or they are very flashy. The females prefer the flashy males, but at the same time those males have a harder time avoiding getting eaten by other, larger fish. Thus both phenotypes are passed on to the next generation of fish.